The deputy ambassador, Thae Yong Ho, has arrived with his family in South Korea, making him the highest-ranking diplomat ever to defect to the South.

Mr Thae defected due to his discontent with the regime and for the future of his child, Jeong Joon-hee, a spokesman at the South's Unification Ministry, told a news conference.

Mr Jeong declined to give details on the timing of Mr Thae's arrival in the South.

"They are currently under government protection and relevant institutions are going ahead with necessary procedures as usual," he said.

According to the South Korean daily newspaper JoongAng Ilbo, the DPRK Embassy made attempts to figure out the diplomat's whereabouts, but failed.

The newspaper said Mr Thae's role at the embassy included keeping track of North Korean defectors in and around London and helping to counter the UK's criticism of North Korea's human rights violations.

He also handled consular services.

The Foreign Office said they were not commenting on the reports and could not confirm if they were trying to verify them.

The Government's list of foreign diplomats in the UK shows there are five diplomats in addition to an ambassador based at the DPRK embassy in Ealing, west London.

North Korea often accuses the South of kidnapping or enticing its citizens to defect.

The North has recently resumed its production of plutonium and says it has no plans of stopping nuclear tests as long as perceived US threats remain.

In a written interview with Kyodo, North Korea's Atomic Energy Institute, which has jurisdiction over its main Yongbyon nuclear facilities, said: "We have reprocessed spent nuclear fuel rods removed from a graphite-moderated reactor."

The institute also said North Korea had been producing highly enriched uranium necessary for nuclear arms and power "as scheduled", Kyodo added.